Everyone saying UCAS doesn't work like that is missing the point that Kate had already accepted a place in 2000 to study at Edinburgh. When William announced he was going to St Andrews and having a gap year, she literally withdrew her application and reapplied for St Andrews the following year. It was a risk as she had no guarantee of a place, but she only did it after she knew William was going to St Andrews (before that people were predicting he'd go to Edinburgh).
That was the point she had to have the grades needed for St Andrew course to get accepted, be prepared to take a gap year probably less an issue for her family with their money - and take a risk she may not get a place - then get in his orbit and be liked by him.
It's far from a fool proof impossible to fail plan orchestrated entirely by her mother to snare the heir to the throne.
Wasn't Mountbatten described as engineer of Queen and Prince Philp's marriage - and Queen mother and maternal GM of Dina behind that marriage - despite that GM having pulled her DD out of school and pushed her to marry into Spencer family after her lodger got engaged to heir - I don't think they've had half the flack directed at them as Carole Middleton has.